Discussion: Spondylolysis is a common clinical condition that can result in low back pain. Patients with spondylolysis have a defect in the pars interarticularis of the neural arch, that portion of the neural arch that connects the superior and inferior articular facets.

Spondylolysis is believed to be caused by repeated microtrauma, resulting in stress fracture of the pars interarticularis. Heredity also is believed to be a factor. Patients with spina bifida occulta have an increased risk for spondylolysis. Approximately 95% of cases of spondylolysis occur at the L5 level. Lyses can occur much less commonly at other lumbar or the thoracic levels. Involvement of multiple levels is rare. The process may be unilateral or bilateral.

Patients with suspected spondylolysis should be evaluated initially with plain radiography, consisting of anteroposterior, lateral, and oblique views of the lumbar spine. The lateral views are most sensitive for detection of pars fractures, and the oblique views are most specific.

If plain radiographs are negative or inconclusive, further imaging may be warranted. MRI, CT, and single-photon emission computed tomography (SPECT) bone scintigraphy are used to further evaluate these patients. Currently, controversy surrounds the designation of one of these tests as most useful in the evaluation of spondylolysis.

Some investigators and practicing radiologists believe that after normal radiographs have been obtained, MRI of the lumbar spine should be next. However, the examination must be performed with thin section images (3 mm) and at relatively high resolution (256 x 192 matrix). Imaging should be obtained in two planes, sagittal and axial. Obtain T1-weighted (short recovery time [TR]/echo time [TE]) and T2-weighted (long TR/TE) with fat suppression images. Usually, this type of MRI examination requires a high field magnet (minimum of 1 T).

CT of the lumbar spine can be performed after obtaining radiographs or after an equivocal MRI. Perform the examination with stacked thin (2 mm) axial sections through the portion of the spine in question. Perform sagittal reconstructions. However, even without reconstructions, pars fractures can be identified on CT because of the absence of a complete ring of bony structures at a given vertebral level.

Nuclear medicine SPECT bone scintigraphy also can be obtained after plain radiographs. The nuclear medicine SPECT examination can provide images in the axial, coronal, and sagittal planes. The spatial resolution of SPECT bone scintigraphy is less than CT or MRI.

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